This invention relates to a combination release coating and barrier coating for use on substrates such as linerless labels. More particularly, the invention relates to a release coating which provides an improved surface for direct thermal printing and good release properties from pressure sensitive adhesives, and to a barrier coating which provides moisture and abrasion resistance to the substrate surface.
The use of linerless labels has become increasingly popular in recent years as such labels do not require a separate release liner, which effectively doubles the number of labels that can be stored on a given roll or stack of labels. Typical linerless labels are disclosed in U.S. Pat. Nos. 4,851,383, 5,508,247 and 5,292,713, and generally comprise a substrate having a thermally imagable coating on one surface which may be activated to form color images. A silicone release composition is coated over the thermally imagable coating, and a pressure sensitive adhesive is included on the second surface of the substrate. The adhesive may be permanent or repositionable. By coating the silicone layer over the thermally imagable layer, the labels may be rolled or stacked without the use of a release liner. The labels may be imaged through the release coating using a thermal printer which activates the thermally imagable coating.
Silicone release coatings currently in use include coatings based on acrylate functional silicones which are cured by free radical polymerization. However, because free radical polymerizations are inhibited by the presence of oxygen, expensive nitrogen inerting is typically employed in order to obtain acceptable cure speeds. Release coatings utilizing UV curable epoxy organo polysiloxanes are also known. However, while these coatings cure rapidly, it is difficult to achieve adequate release properties when using such coatings. For example, the force required for release from the pressure sensitive adhesives must be sufficiently low to allow easy removal of the label but not so low that the labels will release or dispense prematurely. Because silicone release coatings must be formulated to provide adequate release properties, it is often difficult to print and/or write on the release coatings due to low surface energy or high contact angle.
Accordingly, there is still a need in the art for a release coating and method of making linerless thermal labels which provides a good printing surface for printing with direct thermal printers as well as providing good release properties from pressure sensitive adhesives.
The present invention meets that need by providing a release coating for substrates such as linerless labels which permits high quality thermal imaging through the coating and which provides stable release properties. In addition, the present invention provides a barrier coating which provides moisture and abrasion resistance to the label substrates and helps prevent adhesive failure of the pressure sensitive adhesives used on the substrates.
According to one aspect of the present invention, a combination release coating and barrier coating is provided for use on a substrate, where the release coating comprises an epoxy-silicone copolymer, a release modifier and a cationic photoinitiator. Preferably, the release coating comprises from about 90-98% by weight of the epoxy-silicone copolymer, from about 1 to 4% by weight of the release modifier, and from about 1 to 4% by weight of the cationic photoinitiator. Preferably, the epoxy-silicone copolymer comprises a UV curable epoxy-functional siloxane. The cationic photoinitiator is preferably selected from the group consisting of diaryl iodonium tetrakis (pentafluoro phenyl) borate salt, bis(dodecylpheryl) iodonium hexafluoroantimonate, and 4-octadecylphenyl phenyl iodonium hexa fluoroantimonate.
The barrier coating preferably comprises a grafted starch, a grafted styrene resin, poly(vinyl alcohol) and a crosslinking agent. The barrier coating may be applied to the first and/or second surfaces of the substrate. When applied to the first surface, the barrier coating functions to provide moisture and abrasion resistance to the substrate. When applied to the second surface of the substrate, the coating functions to prevent bleed-through of the pressure sensitive adhesive on the substrate which could result in adhesive failure.
A preferred method for making the linerless thermal labels of the present invention comprises the steps of a) providing a web comprising a label substrate having first and second surfaces, with the first surface including a thermally imagable coating; b) applying a barrier coating to the first, second, or both surfaces of the web; c) applying a release coating to the first surface of the web, where the release coating comprises an epoxy-silicone copolymer, a release modifier, and a cationic photoinitiator; and c) applying a pressure sensitive adhesive to the second surface of the web. Preferably, the release coating is cured by UV radiation prior to application of the pressure sensitive adhesive.
Preferably, the barrier coating is applied to the first surface of the web prior to applying the release coating. The barrier coating is preferably applied to the second surface of the web prior to applying the adhesive.
The resulting linerless label exhibits a good printing surface when printed using direct thermal printers, provides stable release properties, and exhibits moisture and abrasion resistance.
Accordingly, it is a feature of the present invention to provide a release coating for substrates such as linerless thermal labels which provides an improved surface for direct thermal printing and good release properties from pressure sensitive adhesives. It is a further feature of the invention to provide a barrier coating which provides moisture and abrasion resistance to the substrate and which prevents adhesive failure when used on a substrate including a pressure sensitive adhesive. Other features and advantages of the invention will be apparent from the following description, the accompanying drawings, and the appended claims.